In certain known paint spray guns the thread that connects the body of the gun and the air nozzle ring is a DIN 13 fine thread. The nominal diameter of the thread is between M 30 and M 38. Pitches of 1.0 mm, 1.5 mm and 1.75 are common. With this type of thread a precise centering of the air nozzle relative to the paint nozzle is possible, which is a precondition of good spray results.
However, the use of fine thread has disadvantages. The best known disadvantage is that when the thread gets dirty, its cleaning is time consuming. A further disadvantage is that at least five revolutions are necessary to screw the air nozzle ring onto the gun body or to unscrew it. Furthermore, cross-threading often occurs when the air nozzle ring is screwed onto the outer thread of the body of the gun, resulting in damage to the thread.
To overcome these disadvantages the present inventor has considered replacing the fine thread with a trapezoid thread. A metric ISO-trapezoid thread is described in DIN 103. However, the inventor has found that a standard trapezoid thread would also have disadvantages. A standard trapezoid thread would require reinforcement of the wall of both the gun body and the air nozzle ring in the area of the trapezoid thread. This would increase the weight of the gun body by approximately 10%, and thus increase the weight of spray guns, increasing the strain on the user. Another potential disadvantage could result from the large flank clearance of a standard trapezoid thread which may cause cross-threading of the air nozzle ring and cause the ring to lean to one side. As a result of this, the centers of the air nozzle and the paint nozzle would not be in alignment, negatively affecting the paint stream. Investigation by the inventor also showed that the substantially larger pitch of the usual trapezoid thread did not make it easier to clean the thread as compared to the fine thread. Thus, it is proposed to modify the standard trapezoid thread to improve its usefulness as a connector of a gun body and a nozzle ring.